Hi,

It is not possible to install third party apps on odoo online instances. If you have odoo enterprise installed in your own premise server or hosted in Odoo.SH, you can use third party apps. But the same not possible with odoo online instance's.

Not sure about the nature of your third party app, mostly it will fail, but have a try.

* install base_import_module app
* now you will get a menu to import app ( this for data files/modules)

* see if it works

Thanks